Recently, the number of metallic carriers used for a catalytic device for purifying the exhaust gas from an internal combustion engine, particularly a gasoline engine of a car, have gradually increased. This is because a metallic carrier has a greater porosity than the ceramic carriers that have been used, and also has higher durability in an environment where temperature changes are great. Generally, the metallic carrier is fabricated by the following procedure: forming a flat foil having a thickness of about 50 .mu.m from a heat-resisting stainless steel foil and then forming a corrugated foil by corrugating the flat foil, winding the flat and corrugated foils or stacking the two foils to form a honeycomb body, placing the honeycomb body in an outer jacket and then mutually bonding the honeycomb body and the jacket.
A Fe-Cr-Al alloy foil having an excellent oxidation resistance has been used for the stainless steel foil as described in Japanese Examined Patent Publication (Kokoku) Nos. 58-23138 and 54-15035 and Japanese Unexamined Patent Publication (Kokai) No. 56-96726. The foil forms an aluminum oxide film on the surface thereof at a high temperature, and maintains an extremely high oxidation resistance. Various bonding methods have been employed, such as the brazing described in Japanese Unexamined Patent Publication (Kokai) No. 61-199574, the resistance welding described in Japanese Unexamined Patent Publication (Kokai) No. 64-40180, the laser and electron beam welding described in Japanese Unexamined Patent Publication (Kokai) No. 54-13462, and various other methods.
The Fe-Cr-Al alloy foil used for the honeycomb body, such as a 20Cr-5Al steel foil, has an excellent oxidation resistance, but its workability is inferior because of high Al content. Accordingly, productivity is extremely low and the production cost is high.
The environment of the practical use of the honeycomb body is about 800.degree. C. at the highest in most cases, and the oxidation resistance of this foil is, in many cases, excessive.
Brazing is most widely used as the bonding method. However, the brazing material used in this method is expensive. Furthermore, the bonding process includes the application of a binder, attachment of the brazing material and vacuum heat-treatment, and is extremely complicated. Resistance welding is a production method having low productivity and is not suitable for mass-production. Laser beam welding involves the problem that the welding apparatus is extremely expensive.
A diffusion bonding is known as a relatively economical bonding method and is described in U.S. Pat. No. 4,300,956 and Japanese Unexamined Patent Publication (Kokai) No. 1-270947. However, in order to secure the oxidation resistance necessary for a catalytic substrate by the foil material, the following problem occurs. Because the material used is an Fe-Cr-Al alloy, it reacts with trace oxygen sources (CO, H.sub.2 O) inside a furnace during vacuum heat-treatment for diffusion bonding, and an extremely thin aluminum oxide film is formed on the foil surface. Accordingly, even when the material is heated to a high temperature above 1,200.degree. C., this film impedes diffusion bonding, and bonding unavoidably becomes local and unstable.
To solve such a problem, technologies requiring a long time for diffusion bonding are known. For example, Examples of Japanese Unexamined Patent Publication (Kokai) No. 2-14747 describe heat-treatment at 1,100.degree. C. for 2 hours and Japanese Unexamined Patent Publication (Kokai) No. 1-266978 describes, in its Examples, heat-treatment at 1,100.degree. C. for 1.5 hours.
Ordinary stainless steels such as SUS430 and SUS410, described as foil materials in Japanese Unexamined Patent Publication (Kokai) No. 1-270947, lack the oxidation resistance necessary as a substrate of a catalyst for an automobile exhaust gas.
The present invention is completed so as to solve the problems described above, and is directed to provide an economical metallic honeycomb body (and a metallic carrier) for a catalyst by using a stainless steel containing a high Si-Cr and a low Al content and a relatively simple bonding method.
It is another object of the present invention to provide a metal honeycomb body for a catalyst having a high oxidation resistance.